If we are to learn from the past and look just north of the this proposed development, it is easy to see that mismanagement of stormwater can lead to costly legal battles and irreversible environmental damage.
The repercussions of having too much waterflow from runoff or stopping the waterflow from runoff with developments that impact Strutt Creek and its journey to Lake Okanagan would be devastating to this biologically sensitive riparian area. These issues are best described in a series of studies on the subject:
Cascadia Biological Services
The Cascadia report consolidated information related to topography, hydrology, sensitive ecosystem and recommended buffers. In their report, Cascadia provided a summary of physical constraints related to slope aspects and identified conservation values and environment findings from the biophysical assessment. Key recommendations relating to the proposed development included limiting clearing required for construction to the minimum and the alignment of roads to follow the natural topography as well as keeping them as narrow as possible.
The proposed CH access road for the large high-density residential development presents alignment concerns in terms of introducing major new infrastructure of a major egress/ingress road through a large amount of the most environmentally sensitive ESA1 land.
Cascade also recommend a detailed stormwater plan for the development should be developed prior to the initiation of the work. Of particular importance is the stormwater generated adjacent to Strutt Creek as sediment input and increases in volume would negatively impact the watercourse. As a result, stormwater control including bioswales, retention ponds etc. should fully be used in order to reduce peak flows and runoff through the developable area. Note- this report was done prior to the Outlook and Benchland Developments and subsequent drainage issues and lawsuits. Urban Systems Preliminary Stormwater Management Plan
This preliminary report dated May 20, 2011 initially outlined the various criteria and guidelines which govern the stormwater management process for the City of Penticton. The study points out that the Subdivisions and Development Bylaw (SD bylaw) requires that all developments covering an area larger than 5 hectares be serviced by both a minor and major drainage system. The minor system manages runoff from the more frequent events, while the major system manages flows when the minor system capacity is exceeded.This Bylaw specifies return periods of 5 years for minor systems and 100 years for major systems for the analysis and design of systems. "Return Periods" relate to the number of years minor or major flooding can be expected to recur.
Urban Systems' report proposed attenuation of post development flows from events with return periods up to 100 years to 5 years predevelopment rates. This is a very significant reduction to which Urban Systems responded with the statement – “Unfortunately there is no long-term set of recorded flows within either Strutt Creek or Randolph Creek." Urban Systems indicated that it is possible to calculate the predevelopment flow using a variety of tools but they acknowledge that this can work in the Developer’s favour by requiring less on-site retention storage, it can also cause off-site problems downstream when adequate drainage routes and infrastructure do not exist.
The Urban Systems ("Urban") report discusses Strutt creek in some detail. They indicated that at the time of their reconnaissance in 2011, no water flow was in the flume. Subsequent to this report, Strutt Creek commenced continuous year-round water flow and in 2018 snowmelt and the high intensity rainstorm flooded and washed out the vineyard road that was described by Urban Systems'.
Figure 3.2 a (below), illustrates that the topography clearly shows the development cells over the Spiller property will ultimately flow into Strutt Creek. Urban Systems' erroneously mentions in the 2011 report that the creek is stable under current flow regime conditions which is governed by snow melt.
Urban also erroneously states that the channel “appears to end” in a constructed pond and this is not true. In fact, the channel collects in a holding pond/wetland, and then continues down through an environmentally protected ravine that flows into Lake Okanagan. The stream gradient is a 4% slope with both banks vegetated with grasses and herbs (artic sedge, beaked rush, horsetail, and buttercup).
Downstream of the rip rap channel there is a natural channel with native vegetation for 123 meters. The stream has a small wetland with cattails that is contiguous with the stream. The gradient decreases to almost flat with a short 10% gradient section that is constricted down into a small valley that is an environmentally protected area with a riparian width of 10 to 30 meters on each bank. The area is dominated by a native vegetation area with Black cottonwood, Water Birch, redosier dogwood, nootka rose, mountain alder, blue elderberry, Oregon grape, poison ivy, snowberry, trembling aspen Saskatoon, stinging nettle, false Solomon seal, and northern bedstraw and is home to an incredible number of birds.
The stream and wetland is habitat for the Great Basin “bull” snake, Northern American Racer snake, deer, lynx, bobcat, bear, coyotes, as well as supporting a diverse number of amphibians including tiger salamander, painted turtle, frogs as well as waterfowl and bluebird, yellow breasted chat, woodpecker, barn owl, screech owl, raptors including osprey, hawk and eagle and a vast variety of insects that are beneficials and support the farming practices of nearby farmers including a beekeeper.
The stream has flowed continuously since 2009, including 2 flood events in 2018 and 2019 that flooded the nearby vineyard.
It is suggested in the Urban report that Strutt Creek catchment draining to Naramata road be diverted to Spring Creek along Three Mile Road. This would have an incredible negative impact on the stream that runs under Naramata road into a wetland and carries on through an environmentally protected area, impacting all the wildlife and vegetation described above.
The repercussions of having too much waterflow from runoff or stopping the waterflow from runoff with developments that impact Strutt Creek and its journey to Lake Okanagan would be devastating to this biological sensitive riparian area.
It is clear that this 2011 Spiller Road Preliminary Stormwater Management Plan as well as the City of Penticton Master Drainage Plan (September 2007) are clearly outdated. Subsequent flooding events in the last 5 years along the Naramata Bench have shown that the indicated potential water drainage volumes of those 2007 and 2011 plans were underestimated at that time. Urban's proposal “to attenuate post development flows from events with return periods up to 100 years to 5 years predevelopment rates does not make realistic sense.
Other Naramata Bench Developments:
Tetra Tech Review and Interpretation of Drainage Related issues in Naramata:
In January 2019 Tetra Tech was engaged by the Province of BC to provide advice on drainage issues in Naramata in the vicinity of the Outlook Development and the Kettle Ridge Development. These developments are high density subdivisions of large expensive homes.
Geology - The area is a similar geological setting to the Spiller Road property comprised of sand and gravel glacial deposits over bedrock. The limited thickness of relatively low conductivity soil coupled with low hydraulic conductivity rock at or near the surface was a key contributor to local scale water management issues.
Tetra Tech’s key findings included the following:
·-The design did not consider infiltration rate;
·-The new development acts to collect and route water from multiple sub-watersheds and then concentrate it. This results in more water than pre-development conditions leading to certain watersheds and overloading the system.
·-It does not appear Ecora, the original contractor, considered conditions prior to the design event occurring.
·- Using high permeable shot rock as infill and leaving it uncovered by either finer material or tarmac has led to more water infiltrating the ditch than design contemplated.
- The calculations used to determine the magnitude of water that would report into the KVR did not appear to have considered the overall catchment area. Interpretation:
- Water tends to collect in local depression formed by naturally infilled north west trending gullies. When bedrock is at or near the surface, the groundwater flow is constrained by the bedrock. This has resulted in observed seepage and related geotechnical issues. - Water would have reached the KVR trail in pre-development condition. However, it would have been at a lower rate, lower magnitude and more diffusely than currently observed. Since no geotechnical issues were previously observed, on a 100-year-old trail, it would appear that the pre-existing system could manage the water reporting to it. By adding water from the septic field and /or the Outlook Development, a tipping point has been reached resulting in the observed issues.
- The conclusion that can be reached from Tetra Tech's report is that “ungoverned stormwater management often leads, eventually, to major public expense in infrastructure to solve flooding or erosion problems, sometimes driven by litigation.”